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Typically the order in which one puts words changes the emphasis or focus on a particular part of a phrase. Putting any given word first calls more attention it. So, the best way to phrase this depends partly on that. You could even put "Rules" first:
Rules of Association which are Reduced
But that gets so wordy as to obfuscate the meaning. For the clearest and most formal sounding, I would go with "Reduction of Association Rules". If some description of the specific type of association is useful: "Reduction of [insert adjective here] Association Rules".